Blast celaning method and method and device for producing solid carbon dioxide to be used for the same

ABSTRACT

The present invention provides a blast cleaning method with cleaning performance adjustable by changing the hardness, shape and size of solid carbon dioxide to be used as a shot material, and a method and a device for producing solid carbon dioxide to be used for the blast cleaning method. 
     The blast cleaning method includes the steps of: forming plate-shaped solid carbon dioxide, crushed pieces of which are used as a shot material (Step S 10 ), transporting the formed plate-shaped solid carbon dioxide to a site of work (Step S 20 ), crushing the plate-shaped carbon dioxide at the site of work, etc. (Step S 30 ), putting the crushed pieces into a cleaning device (blasting machine) (Step S 40 ), and cleaning an object to be cleaned by blowing the crushed pieces to the object to be cleaned (Step S 50 ).

TECHNICAL FIELD

The present invention relates to a blast cleaning method using crushedpieces of solid carbon dioxide as a shot material, and specifically, toa blast cleaning method with cleaning performance adjustable by changingthe hardness and size of the shot material, and a method and a devicefor producing solid carbon dioxide to be used for the blast cleaningmethod.

BACKGROUND ART

Blast cleaning is a method for removing rust and coating on an object tobe cleaned made of a metal or ceramic, etc., by blowing a shot materialthereto at a high speed, and generally, sand is frequently used as theshot material. However, when sand is used as the shot material, the shotmaterial easily remains on the object to be cleaned, and an operation torecover the shot material may become necessary. Usually, such a shotmaterial is hardly recovered. Further, the increase in the number ofprocesses increases the cleaning cost. Therefore, in recent years, ablast cleaning method using dry ice as the shot material has receivedattention as a cleaning method in which a shot material hardly remainson the object to be cleaned, and if the shot material remains, it can beeasily recovered. Several inventions and utility models relating to thisblast cleaning method have already been disclosed.

For example, Patent document 1 discloses an invention titled “cleaningdevice, cleaning method, and method for producing solid carbon dioxideparticles” relating to a cleaning device capable of cleaning alarge-area target through a simple cleaning process and a method forproducing dry ice to be used for the cleaning device.

The “cleaning device” as an invention disclosed in Patent document 1includes a first flow channel in which carbon dioxide gas flows, asecond flow channel in which an inert gas flows, a junction for joiningthe gases flowing in the first flow channel and the second flow channel,and a nozzle to be connected to the junction.

With this structure, dry ice blown to a target sublimes and becomescarbon dioxide, so that it is easily removed by an exhaust means, etc.Therefore, the cleaning process becomes simple. By changing thedirection of the nozzle, particles (dry ice) of solid-state carbondioxide formed in the junction can be jetted in a desired direction.Accordingly, a large-area target, etc., can be cleaned.

Patent document 2 discloses an invention relating to “a method forproducing dry ice aerosol” by which micro-ordered or smaller dry iceparticles with less impurities mixed and high hardness can be produced.

In the “method for producing dry ice aerosol” as an invention disclosedin Patent document 2, liquefied carbon dioxide is decompressed to turninto a gas-liquid mixed state and then jetted into the inside of anozzle, and into this nozzle, nitride gas is jetted at a high speed.

This production method has an operation in which liquefied carbondioxide in a gas-liquid mixed state and carbon dioxide are supercooledinside the nozzle by the cold of the nitrogen gas, and aerosol includingthe nitrogen gas as a dispersion medium and dry ice particles as adispersion phase is formed. That is, according to the method of thisinvention, the shot material does not remain on the work, andmicro-ordered or smaller dry ice particles with less impurities mixedand high hardness can be formed.

Further, Patent document 3 discloses an invention titled “dry ice snowjet cleaning device and cleaning method” relating to a cleaning devicecapable of efficiently removing burrs of plastic moldings and extraneousmatter on precision parts, etc., and a cleaning method using thecleaning device.

The “dry ice snow jet cleaning device” as an invention disclosed inPatent document 3 includes a dry ice snow jet cleaning nozzle, aliquefied carbon dioxide container, a jet gas source, a dry ice snowforming and supply system which connects the liquefied carbon dioxidecontainer and the jet cleaning nozzle, an opening and closing means forintermittently supplying liquefied carbon dioxide provided in this dryice snow forming and supply system, and a jet gas supply system whichconnects the jet gas source and the jet cleaning nozzle.

The “dry ice snow jet cleaning device” thus structured has an operationin which a shot material does not remain on a work. By intermittentlyjetting dry ice snow while continuously jetting a jet gas as apropellant gas, an object to be cleaned can be cleaned without cooling.Accordingly, condensation can be prevented and uniform and satisfactorycleaning can be attained.

Patent document 1: Japanese Published Unexamined Patent Application No.2004-89944Patent document 2: Japanese Published Unexamined Patent Application No.2003-54929Patent document 3: Japanese Published Unexamined Patent Application No.2001-277116

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

However, in the invention disclosed in Patent document 1 as aconventional technique described above, there is a possibility thatimpurities mix when carbon dioxide is sublimed, and the hardness of dryice to be formed becomes lower. Powdery dry ice is not angular, so thatits cleaning performance is low. Further, the hardness, shape, and sizeof dry ice are not changeable, so that the required cleaning performancecannot be realized.

In the invention disclosed in Patent document 2, formed dry iceparticles are not angular, so that sufficient hardness may not beobtained. Further, the hardness, shape and size of a shot material arenot adjustable, so that the cleaning performance is limited.

In the invention disclosed in Patent document 3, dry ice snow is notangular, so that its cleaning performance cannot be increased. Further,the hardness, shape, and size of dry ice snow are not changeable, sothat its cleaning performance is not adjustable.

The present invention was made in view of the conventional circumstancesdescribed above, and an object thereof is to provide a blast cleaningmethod the cleaning performance of which can be adjusted by changing thehardness, shape, and size of solid carbon dioxide to be used as a shotmaterial, and a method and a device for producing solid carbon dioxideto be used for the blast cleaning method.

Means for Solving the Problems

In order to achieve the above-described object, a blast cleaning methodaccording to a first aspect of the invention, in which a shot materialis blown to an object to be cleaned at a high speed to separateextraneous matter, includes the steps of: forming plate-shaped solidcarbon dioxide by cooling liquid carbon dioxide while applying apressure higher than the pressure of the triple point thereto; crushingthe plate-shaped solid carbon dioxide; and cleaning an object to becleaned by using the crushed solid carbon dioxide as a shot material.

This blast cleaning method has an operation in which the cleaningperformance is determined according to the hardness, shape, and size ofsolid carbon dioxide. The plate-shaped solid carbon dioxide is moredifficult to vaporize than pellet-like dry ice.

According to a second aspect of the invention, in the blast cleaningmethod set forth in claim 1, the thickness of the plate-shaped solidcarbon dioxide is 3 to 8 millimeters.

According to this blast cleaning method, the thickness of theplate-shaped solid carbon dioxide is not less than 3 millimeters, sothat the minimum necessary mass as a shot material is secured. Further,the thickness of the plate-shaped solid carbon dioxide is not more than8 millimeters, so that it is easily crushed.

A method for producing solid carbon dioxide according to a third aspectof the invention is a method for producing solid carbon dioxide to beused as a shot material in the blast cleaning method set forth in claim1 or 2, and includes a first cooling step of cooling the liquid carbondioxide to a sublimation temperature at the atmospheric pressure from atemperature higher than the temperature of the triple point whileapplying a pressure higher than the pressure of the triple pointthereto; and a second cooling step of cooling solid carbon dioxideformed in the first cooling step to a temperature lower than thesublimation temperature.

This method for producing solid carbon dioxide has an operation in whichliquid carbon dioxide is solidified while impurities (gases other thancarbon dioxide) are expelled. Further, this method has an operation inwhich the hardness of solid carbon dioxide formed in the first coolingstep is increased in the second cooling step. Specifically, throughthese steps, solid carbon dioxide much harder than in the conventionalmethod in which pressurized liquid carbon dioxide is released to theatmosphere and solidified is formed. Further, the method has anoperation in which the hardness of solid carbon dioxide to be formed isadjusted by changing a cooling end temperature in the second coolingstep. Numerous cracks occur in solid carbon dioxide cooled to atemperature lower than the sublimation temperature at the atmosphericpressure due to a temperature difference when it is exposed to theatmosphere. Accordingly, sharp angles are formed when crushingregardless of the sizes of crushed pieces.

A device for producing solid carbon dioxide according to a fourth aspectof the invention is a device for producing solid carbon dioxide to beused as a shot material in the blast cleaning method of the first orsecond aspect of the invention, and includes a rectangular tabularhollow part formed to become capable of being filled with liquid carbondioxide; a cooling jacket formed so as to surround this hollow part; avacuum heat insulating chamber formed so as to surround the coolingjacket; a vacuum pump for vacuuming the vacuum heat insulating chamberand the hollow part; a liquid carbon dioxide supply means for supplyingliquid carbon dioxide into the hollow part through a first supply pipe;a refrigerating machine to be connected to the cooling jacket via asecond supply pipe; a first on-off valve installed in the first supplypipe; and a second on-off valve installed in the second supply pipe,where the hollow part, the cooling jacket, and the vacuum heatinsulating chamber form a vacuum container with a triplex structure.

The device for producing solid carbon dioxide thus structured has anoperation in which gases which may become impurities when forming solidcarbon dioxide are discharged by the vacuum pump from the hollow partbefore injecting liquid carbon dioxide. Further, the device has anoperation in which liquid carbon dioxide injected into the hollow partis cooled by heat exchange with a coolant injected into the coolingjacket via the second supply pipe. Further, the device has an operationin which heat transfer between the hollow part and the cooling jacketand the outside is shut off by the vacuum heat insulating chamberdecompressed to a vacuum state by the vacuum pump. Additionally, thecoolant at a temperature adjusted by using the refrigerating machine isinjected into the cooling jacket, so that the temperature of the hollowpart is easily controlled.

Effects of the Invention

As described above, according to the blast cleaning method of the firstaspect of the invention, the cleaning performance can be adjusted bychanging the hardness, shape, or size of the shot material. The amountof reduction caused by vaporization during transportation can be madesmaller than in the case where the pellet-like dry ice is transported.Further, the hardness when solid carbon dioxide is formed can bemaintained until just before it is crushed at the site of work.

When powdered dry ice is pelletized, a large-sized hydraulic compressoris required, however, according to the blast cleaning method of thesecond aspect of the present invention, crushed pieces with a desiredsize can be easily formed by a small-sized crushing machine. Therefore,the cost necessary for the cleaning operation can be greatly reduced.

According to the method for producing solid carbon dioxide of the thirdaspect of the present invention, solid carbon dioxide having a hardnesssuitable as a shot material in the blast cleaning method can be easilyformed. Further, plate-shaped solid carbon dioxide from which crushedpieces having sharp angles suitable as a shot material in the blastcleaning method can be formed can be easily produced.

In the device for producing solid carbon dioxide according to the fourthaspect of the present invention, liquid carbon dioxide can be easilycooled to a desired temperature in a state where a pressure higher thanthe pressure of the triple point is applied thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a blast cleaning method of an embodiment of thepresent invention;

FIG. 2A is an external view of an example of a device for producingsolid carbon dioxide of the embodiment of the present invention, andFIG. 2B is a schematic view showing solid carbon dioxide taken out ofthe production device of FIG. 2A;

FIG. 3A is a front view of a vacuum container shown in FIG. 2A, and FIG.3B is a sectional view on the arrow X-X of FIG. 3A;

FIG. 4 is a flowchart of a method for producing solid carbon dioxide ofthe present example;

FIG. 5 is a schematic view of an example of a crushing machine forcrushing plate-shaped solid carbon dioxide; and

FIG. 6A is a front view of a rotary blade of the crushing machine ofFIG. 5, and FIG. 6B is a partial enlarged view of the vicinity of an endportion of FIG. 6A.

DESCRIPTION OF REFERENCE NUMERALS

-   1: vacuum container-   1 a: opening-   2: liquid carbon dioxide-   3: storing container-   4: carbon dioxide solidification chamber-   4 a: opening-   5: cooling jacket-   5 a: coolant supply port-   5 b: coolant discharge port-   6: vacuum heat insulating chamber-   6 a: air inlet-   6 b: drain outlet-   7: solid carbon dioxide-   8: cover-   8 a: supply port-   8 b: discharge port-   9 a: supply pipe-   9 b: vacuuming line pipe-   10: pressure gauge-   10 a: branch pipe-   11 a: coolant supply pipe-   11 b: coolant discharge pipe-   12 a: intake pipe-   12 b: drain pipe-   13 a: on-off valve-   13 b: on-off valve-   13 c: on-off valve-   13 d: on-off valve-   13 e: on-off valve-   14: surface thermometer-   15: crack line-   16: crushing blade-   17: crushing gear-   18: drive shaft-   19: crushed pieces-   20: vacuum pump-   A: arrow-   B: arrow-   C: arrow

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, examples of a blast cleaning method and a method and adevice for producing solid carbon dioxide to be used for the blastcleaning method according to a best embodiment of the present inventionwill be described.

Example

An example of a blast cleaning method will be described with referenceto FIG. 1.

FIG. 1 is a flowchart of the blast cleaning method of the embodiment ofthe present invention.

The blast cleaning method of the present example uses crushed pieces ofsolid carbon dioxide as a shot material, and includes the steps of, asshown in FIG. 1, forming plate-shaped solid carbon dioxide, crushedpieces of which are used as a shot material (Step S10), transporting theformed plate-shaped solid carbon dioxide to a site of work (Step S20),crushing the plate-shaped carbon dioxide at the site of work, etc. (StepS30), and putting the crushed pieces into a cleaning device (blastingmachine) (Step S40), and then cleaning an object to be cleaned byblowing the crushed pieces to the object to be cleaned (Step S50).

First, at Step S10, plate-shaped solid carbon dioxide is formed bycooling liquid carbon dioxide while applying a pressure higher than thepressure of the triple point thereto in advance in a plant, etc.,according to the method described later. If the plate is thinner than 3millimeters, when the plate is crushed, the minimum necessary mass as ashot material cannot be obtained. On the other hand, when the plate isthicker than 8 millimeters, it takes time to crush the plate into a sizesuitable as a shot material, and depending on the circumstances, newsteps become necessary. Further, a great burden is imposed on thecrushing machine, so that this is not preferable. Therefore, in thepresent example, the thickness of the plate-shaped solid carbon dioxideto be formed at Step S10 is set especially to 3 to 8 millimeters. AtStep S20, the formed plate-shaped solid carbon dioxide is transported tothe site of actual work of cleaning while it is housed in a commerciallyavailable cool box, etc. The cool box preferably has a structure capableof housing a plurality of plates of plate-shaped solid carbon dioxidetogether although the structure is not especially limited. This step isnot necessary when the cooling operation is subsequently performed atthe place where the plate-shaped solid carbon dioxide was formed. AtStep S30, the plate-shaped solid carbon dioxide is crushed according tothe method described later to form 1.5-mm-square to 8-mm-square crushedpieces. The crushed pieces are put into a cleaning device (blastingmachine) at Step S40. Then, at Step S50, the cleaning device is operatedto blow a shot material formed of the crushed pieces of solid carbondioxide to an object to be cleaned at a high speed. Accordingly, theobject to be cleaned is cleaned.

This blast cleaning method has an operation in which the cleaningperformance is determined according to the hardness, shape, and size ofthe solid carbon dioxide. The thickness of the plate-shaped solid carbondioxide is not less than 3 millimeters, so that the minimum necessarymass as a shot material is secured. Further, the thickness of theplate-shaped solid carbon dioxide is not more than millimeters, so thatit can be easily crushed. In addition, the method has an operation inwhich the plate-shaped solid carbon dioxide is more difficult tovaporize than pellet-like dry ice.

Specifically, according to the blast cleaning method of the presentexample, the cleaning performance can be adjusted by changing thehardness, shape, or size of the shot material. In addition, the amountof reduction caused by vaporization during transportation can be mademuch smaller than pellet-like dry ice. Further, the hardness when solidcarbon dioxide is formed can be maintained until just before it iscrushed at a site of work, etc., in actuality. Usually, when powdery dryice is pelletized, a large-sized hydraulic compressor is required,however, in the blast cleaning method of the present example, crushedpieces with a desired size can be easily formed with a large-sizedcrushing machine. Therefore, the cost necessary for the cleaningoperation is greatly reduced. Further, the thickness of the plate-shapedsolid carbon dioxide to be formed is not more than 8 millimeters, sothat the energy required for cooling can be reduced more than in theconventional method, and liquid carbon dioxide can be solidifiedreliably up to the center of the thickness. Accordingly, plate-shapedsolid carbon dioxide with uniform hardness is formed.

Next, the step of forming plate-shaped solid carbon dioxide (Step S1 ofFIG. 1) will be described with reference to FIGS. 2 to FIG. 4.

FIG. 2A is an external view of an example of a device for producingsolid carbon dioxide of an embodiment of the present invention, and FIG.2B is a schematic view showing solid carbon dioxide taken out of theproduction device of FIG. 2A. FIG. 3A is a front view of a vacuumcontainer shown in FIG. 2A, and FIG. 3B is a sectional view on the arrowX-X.

The device for producing solid carbon dioxide of the present exampleforms plate-shaped solid carbon dioxide by cooling liquid carbon dioxideinjected by a pressure higher than the pressure of the triple point byheat exchange with a coolant fed into the cooling jacket from therefrigerating machine.

As shown in FIG. 2A and FIGS. 3, the device for producing solid carbondioxide of the present example includes a vacuum container 1, a storingcontainer 3 for liquid carbon dioxide 2, a refrigerating machine (notshown) which controls the temperature of the coolant, and a vacuum pump20. The vacuum container 1 has a triplex structure with pressureresistance consisting of a carbon dioxide solidification chamber 4, acooling jacket 5, and a vacuum heat insulating chamber 6. The carbondioxide solidification chamber 4 is a rectangular tabular hollow parthaving an opening 4 a formed in an upper surface, and outside the carbondioxide solidification chamber 4, the cooling jacket 5 and the vacuumheat insulating chamber 6 are successively provided so as to surroundfive surfaces except for the opening 4 a. The inner surface of thecarbon dioxide solidification chamber 4 is tapered by making the lowerdimension smaller than the upper dimension so that the solidified carbondioxide is easily taken out. For taking-out plate-shaped solid carbondioxide 7 (see FIG. 2B) formed in the carbon dioxide solidificationchamber 4, an opening 1 a is formed in the upper surface of the vacuumcontainer 1 to communicate with the opening 4 a of the carbon dioxidesolidification chamber 4. Further, to the opening 1 a of the vacuumcontainer 1, a cover 8 is attached so as to airtightly close the carbondioxide solidification chamber 4. To a supply port 8 a and a dischargeport 8 b formed in the cover 8, a supply pipe 9 a for liquid carbondioxide 2 and a vacuuming line pipe 9 b are connected, respectively. Tothe supply pipe 9 a, the storing container 3 is connected, and to thevacuuming line pipe 9 b, the vacuum pump 20 is connected. That is, theliquid carbon dioxide 2 is supplied into the carbon dioxidesolidification chamber 4 from the storing container 3 through the supplypipe 9 a, and can be decompressed by the vacuum pump 20.

A coolant supply pipe 11 a and a coolant discharge pipe 11 b areconnected to the coolant supply port 5 a and the coolant discharge port5 b formed in the lower surface of the vacuum container 1 so as tocommunicate with the cooling jacket 5, and to the coolant supply pipe 11a, a refrigerating machine (not shown) is connected. That is, astructure in which a coolant adjusted to a desired temperature by therefrigerating machine is supplied to the cooling jacket 5 through thecoolant supply pipe 11 a, is formed. Further, to an air inlet 6 a and adrain outlet 6 b formed in the upper surface and the lower surface ofthe vacuum container 1, respectively, so as to communicate with thevacuum heat insulating chamber 6, an intake pipe 12 a and a drain pipe12 b are connected, respectively, and to the intake pipe 12 a, thevacuum pump 20 is connected. Further, on-off valves 13 a to 13 d areattached to the supply pipe 9 a, the vacuuming line pipe 9 b, thecoolant discharge pipe 11 b, and the drain pipe 12 b, respectively, inthe vicinities of the supply port 8 a, the discharge port 8 b, thecoolant discharge port 5 b, and the drain outlet 6 b. To a branch pipe10 a branched from the supply pipe 9 a between the on-off valve 13 a andthe supply port 8 a, a pressure gauge 10 is connected via the on-offvalve 13 e. On the upper surface of the vacuum container 1, a surfacethermometer 14 for measuring the temperature of the outer surface of thecarbon dioxide solidification chamber 4 is provided.

The device for producing solid carbon dioxide thus structured has anoperation in which gases which may become impurities when forming solidcarbon dioxide 7 are discharged by the vacuum pump 20 from the carbondioxide solidification chamber 4 before liquid carbon dioxide 2 isinjected. The device has an operation in which liquid carbon dioxide 2injected into the carbon dioxide solidification chamber 4 from thestoring container 3 through the supply pipe 9 a is cooled by heatexchange with a coolant injected into the cooling jacket 5 through thecoolant supply pipe 11 a. The device has an operation in which thevacuum heat insulating chamber 6 decompressed to a vacuum state by thevacuum pump 20 shuts off heat transfer between the carbon dioxidesolidification chamber and cooling jacket 5 and the outside. Further,the device has an operation in which the cooling rate of the liquidcarbon dioxide 2 in the carbon dioxide solidification chamber 4 iscontrolled by adjusting the temperature of the coolant by using therefrigerating machine.

That is, according to the device for producing solid carbon dioxide ofthe present invention, liquid carbon dioxide 2 can be easily cooled to adesired temperature in a state where a pressure higher than the pressureof the triple point is applied thereto.

Next, a method for producing solid carbon dioxide by using theproduction device structured as described above will be described withreference to FIG. 4.

FIG. 4 is a flowchart of the method for producing solid carbon dioxideof the present example.

As shown in FIG. 4, at Step S11, the vacuum pump 20 is actuated todecompress the vacuum heat insulating chamber 6 to a degree of vacuum of10⁻³ MPa. At Step S12, the vacuum pump 20 connected to the dischargepipe 9 b is actuated to decompress the carbon dioxide solidificationchamber 4 to a degree of vacuum of 10⁻³ MPa, and then, a coolant isinjected into the cooling jacket 5 to cool the carbon dioxidesolidification chamber 4 until the surface thermometer 14 stands atapproximately −50° C. Next, at Step S13, the on-off valve 13 a isoperated to inject the liquid carbon dioxide 2 inside the supply pipe 9a into the carbon dioxide solidification chamber 4 in a state where apressure higher than the pressure (0.518 MPa) of the triple point isapplied thereto. At Step S14, after confirming that the carbon dioxidesolidification chamber 4 is completely filled with the liquid carbondioxide 2, the on-off valve 13 a is closed to shut and airtightly closethe carbon dioxide solidification chamber 4. Then, the coolant at atemperature controlled by the refrigerating machine is fed into thecooling jacket 5 to cool the liquid carbon dioxide 2 in a state where apressure higher than the pressure of the triple point is appliedthereto, and the temperature of the carbon dioxide solidificationchamber 4 is lowered from −50° C. to −79° C. by 1° C. per approximatelyone minute. Specifically, at Step S14, by cooling the liquid carbondioxide 2 from a temperature higher than the temperature (−56.6° C.) ofthe triple point to approximately the sublimation temperature (−78.9°C.) at the atmospheric pressure while applying a pressure higher thanthe pressure of the triple point thereto, the liquid carbon dioxide 2 isdirectly solidified without being vaporized. An object of this step isto solidify the liquid carbon dioxide 2 while impurities (gases otherthan carbon dioxide) inside the carbon dioxide solidification chamber 4are expelled. Next, at Step S15, the temperature of the carbon dioxidesolidification chamber 4 is lowered from −79° C. to −100° C. in a shorttime. Through this step, the hardness of the solidified liquid carbondioxide 2 is further increased. By changing the above-described coolingend temperature (−100° C.), the hardness of the solid carbon dioxide 7to be formed is easily adjusted. At Step S16, after confirming that thesolidification of the liquid carbon dioxide 2 has been completed and thepressure of the carbon dioxide solidification chamber 4 has been loweredto the atmospheric pressure, the cover 8 of the vacuum container 1 isopened. Thereafter, the solid carbon dioxide 7 having a surface slightlymolten is taken out of the vacuum container 1. At this time, thetemperature of the solid carbon dioxide 7 is not more than thesublimation temperature at the atmospheric pressure. This temperaturedifference causes numerous crack lines 15 on the solid carbon dioxide 7(see FIG. 2B).

According to this method for producing solid carbon dioxide, impuritiesare hardly mixed when the liquid carbon dioxide 2 is solidified, so thatsolid carbon dioxide 7 much harder than in the conventional method inwhich pressurized liquid carbon dioxide 2 is released to the atmosphereand solidified is formed, is formed. Further, numerous crack lines 15are caused on the plate-shaped solid carbon dioxide 7 taken out of thevacuum container 15, so that when it is crushed, sharp angles are formedregardless of the sizes of the crushed pieces.

That is, according to the method for producing solid carbon dioxide, thesolid carbon dioxide 7 having a hardness suitable as a shot material inthe blast cleaning method can be easily formed. Further, plate-shapedsolid carbon dioxide from which crushed pieces having sharp anglessuitable as a shot material in the blast cleaning method can be formedcan be easily produced.

The method for producing solid carbon dioxide of the present inventionis not limited to the present example. Specifically, the cooling starttemperature of the liquid carbon dioxide 2 at Step S14 is not limited to−50° C. However, to prevent the liquid carbon dioxide 2 from beingsolidified when it is injected into the carbon dioxide solidificationchamber 4, this cooling start temperature must be set to be higher thanat least the temperature of the triple point. However, if the coolingstart temperature is excessively higher than the temperature of thetriple point, the time for this cooling step becomes longer. Therefore,the cooling start temperature is preferably higher than the temperatureof the triple point and close to the temperature of the triple point.The cooling end temperature must be set to be substantially equal to thesublimation temperature at the atmospheric pressure so that, even if thecooling step of Step S15 is omitted and the solid carbon dioxide 7formed at Step S14 is taken out of the vacuum container 1 and exposed tothe atmosphere, it does not sublimate immediately. Further, in thepresent example, the temperature of the carbon dioxide solidificationchamber 4 is lowered by 1° C. per approximately one minute at Step S14,however, the cooling rate of this step is not limited to this, but canbe variously changed. However, this step is preferably slowly performedin a comparatively long time so that impurities (gases other than carbondioxide) inside the carbon dioxide solidification chamber 4 can beefficiently expelled. The cooling end temperature of Step S15 is set tobe lower than at least the sublimation temperature at the atmosphericpressure, and not limited to −100° C. shown in this example. In otherwords, the cooling end temperature is changeable as appropriateaccording to the hardness of the solid carbon dioxide 7 to be formed.Further, the cooling time is also changeable as appropriate. Consideringthat the hardness of the solid carbon dioxide 7 is hardly influenced byan increase in the cooling time of this step and considering the coolingefficiency, the cooling time is preferably comparatively short.

A step of crushing the plate-shaped solid carbon dioxide (Step S30 ofFIG. 1) will be described in detail with reference to FIG. 5 and FIG. 6.

FIG. 5 is a schematic view of an example of a crushing machine forcrushing plate-shaped solid carbon dioxide. FIG. 6A is a front view of arotary blade of the crushing machine of FIG. 5, and FIG. 6B is a partialenlarged view of the vicinity of an end portion of FIG. 6A.

As shown in FIG. 5, the crushing machine of this example includes a pairof crushing gears 17 and 17 having a plurality of crushing blades 16.The crushing gears 17 and 17 are attached to drive shafts 18 and 18 sothat their crushing blades 16 engage with each other. That is, thecrushing gears 17 and 17 rotate in the directions of the arrows A and Baccording to rotations of the drive shafts 18 and 18, respectively. Inthis state, when the plate-shaped solid carbon dioxide 7 is put in thedirection of the arrow C between the crushing gears 17 and 17, the solidcarbon dioxide 7 is crushed by shearing of the crushing blades 16 and 16while being pushed-in between the two drive shafts 18 and 18 accordingto the rotations of the crushing gears 17 and 17. Accordingly, crushedpieces 19 of the solid carbon dioxide 7 are formed. The drive shafts 18and 18 are set parallel to each other and changeable in distance to eachother. Further, the crushing gears 17 and 17 can be attached to thedrive shafts 18 and 18 so that the crushing blades 16 and 16 engage witheach other asymmetrically with respect to the solid carbon dioxide 7 tobe put therebetween.

As shown in FIG. 6A and FIG. 6B, in the crushing machine of the presentexample, a plurality of crushing gears (with a diameter of 40millimeters and a thickness of 3 millimeters) having 25 crushing bladesare attached to the drive shafts 18. The crushing gears 17 adjacent toeach other are attached to the drive shafts 18 so that their crushingblades 16 do not overlap each other as viewed in the directions of thedrive shafts 18. In this case, the sizes of the crushed pieces 19 becomenot more than 6 millimeters×4.9 millimeters, theoretically. When aspacer with a thickness of 1 millimeter is provided between the crushinggears 17, crushed pieces 19 with a width of 5 to 8 millimeters areobtained.

The crushing machine thus structured has an operation in which the sizesof the crushing pieces 19 change according to the interval between thedrive shafts and 18. Specifically, when the interval between the driveshafts 18 is narrowed, small crushed pieces 19 are formed, and when theinterval between the drive shafts 18 is widened, large crushed pieces 19are formed. Further, the crushing machine has an operation in which thesizes of the crushed pieces 19 change according to the width of thespacer set between the two crushing gears 17. Further, the sizes of thecrushed pieces 19 are also changed by attaching the crushing gears 17and 17 to the drive shafts and 18 so that the crushing gears engage witheach other asymmetrically with respect to the solid carbon dioxide 7 puttherebetween.

As described above, in the crushing machine of the present example, thesizes of the crushed pieces 19 to be formed by crushing the plate-shapedsolid carbon dioxide 7 can be adjusted.

The plate-shaped solid carbon dioxide 7 produced according to thismethod can be used not only as a shot material for a blast cleaningmethod, but also be used for the same purpose as conventionalcommercially available squared dry ice. The size (except for thethickness) of the plate-shaped solid carbon dioxide is not limited tothe size shown in the present example, but can be changed asappropriate. Further, as a method for cooling the container whenproducing plate-shaped solid carbon dioxide 7, a method in which thecontainer is immersed in a coolant, a method in which a coolant pipe isbrought into contact with the container, and a method in which thecontainer is directly attached to a cooling machine, etc., areavailable, and the method is not especially limited to the coolingmethod shown in the present example.

INDUSTRIAL APPLICABILITY

As described above, the invention of claims 1 to 4 of the presentinvention are applicable to the field of blast cleaning in whichextraneous matter is separated from an object to be cleaned by blowing ashot material to the object to be cleaned at a high speed.

1. A blast cleaning method in which a shot material is blown to an object to be cleaned at a high speed to separate extraneous matter, comprising the steps of: forming plate-shaped solid carbon dioxide (7) by cooling liquid carbon dioxide (2) while applying a pressure higher than the pressure of the triple point thereto; crushing the plate-shaped solid carbon dioxide (7); and cleaning an object to be cleaned by using the crushed solid carbon dioxide (7) as a shot material.
 2. The blast cleaning method according to claim 1, wherein the thickness of the plate-shaped solid carbon dioxide (7) is 3 to 8 millimeters.
 3. A method for producing solid carbon dioxide (7) to be used as a shot material in the blast cleaning method according to claim 1 or 2, comprising: a first cooling step of cooling liquid carbon dioxide (2) to a sublimation temperature at the atmospheric pressure from a temperature higher than the temperature of the triple point while applying a pressure higher than the pressure of the triple point thereto; and a second cooling step of cooling the solid carbon dioxide (7) formed in the first cooling step to a temperature lower than the sublimation temperature.
 4. A device for producing solid carbon dioxide (7) to be used as a shot material in the blast cleaning method according to claim 1 or 2, comprising: a rectangular tabular hollow part (4) formed to become capable of being filled with liquid carbon dioxide (2); a cooling jacket (5) formed so as to surround this hollow part (4); a vacuum heat insulating chamber (6) formed so as to surround the cooling jacket(5); a vacuum pump (20) for vacuuming the vacuum heat insulating chamber (6) and the hollow part (4); a liquid carbon dioxide supply means (3) for supplying the liquid carbon dioxide (2) into the hollow part (4) through a first supply pipe (9 a); a refrigerating machine to be connected to the cooling jacket (5) via a second supply pipe (11); a first on-off valve (13 a) installed in the first supply pipe (9 a); and a second on-off valve (8 b) installed in the second supply pipe, wherein the hollow part (4), the cooling jacket (5), and the vacuum heat insulating chamber (6) form a vacuum container (1) with a triplex structure. 